True Altitude

True altitude is the actual height of the aircraft above mean sea level (MSL). It differs from indicated altitude (the altimeter reading) when the temperature departs from the International Standard Atmosphere (ISA). In cold air, the altimeter over-reads — true altitude is lower than indicated altitude.

True Altitude and Cold Temperature

In cold air, air is denser than standard, and the actual pressure at a given altitude is higher than it would be at ISA. The altimeter “thinks” it is at a higher altitude than it actually is. For operations in Canada, where temperatures well below ISA are common in winter, this error can be substantial — at ISA −30°C, an aircraft indicating 5,000 ft may actually be closer to 4,000 ft.

Calculating True Altitude

True altitude ≈ Indicated altitude + [(Indicated altitude − Station elevation) × (ISA deviation / 273)]. More practically: apply the ICAO cold temperature altitude correction table, which gives the altitude correction for various temperatures and heights above station. This correction is mandatory for IFR approaches at Canadian airports with published cold temperature corrections.

Related Terms

See also: Indicated Altitude, Pressure Altitude, Density Altitude.